In such a radar level gauge known from U.S. Pat. No. 6,950,055 (see there FIGS. 6 and 11), the dielectric barrier comprises a thin insulating layer which non-conductively separates said first and second waveguide members. Correspondingly, the width of the gap between the waveguide members is equal to the thickness of the layer. As the insulating layer serves as a dielectric window, its optimal thickness corresponds approximately to a half wavelength of the microwaves. The U.S. Pat. No. 6,950,055 states a thickness of e.g. 0.5 mm and that the design around the insulating layer must be well designed in order to avoid disturbances in the microwave function. As the insulating layer is normally too thin to mechanically seal against the measuring environment the antenna is directed to, an additional sealing in form of a shaft member can be inserted into the first waveguide member leading to the antenna (FIG. 10 of U.S. Pat. No. 6,950,055).